Metal-hydroxyls mediate intramolecular proton transfer in heterogeneous O-O bond formation

Abstract

Metal (hydro)oxides are among the most effective heterogeneous water oxidation catalysts. Elucidating the interactions between oxygen-bridged metal sites at a molecular level is essential for developing high-performing electrocatalysts. Here we demonstrate that adjacent metal-hydroxyl groups function as intramolecular proton-electron transfer relays to enhance water oxidation kinetics. We achieved this using a well-defined molecular platform with an aza-fused pi-conjugated microporous polymer that coordinates molecular Ni or Ni-Fe sites that emulate the structure of the most active edge sites in Ni-Fe materials for studying the heterogeneous water oxidation mechanism. We combine experimental and computational results to reveal the origin of pH-dependent reaction kinetics for O-O bond formation. We find both the anions in solution and the adjacent Ni3+-OH site act as proton transfer relays, facilitating O-O bond formation and leading to pH-dependent water oxidation kinetics. This study provides significant insights into the critical role of electrolyte pH in water oxidation electrocatalysis and enhancement of water oxidation activity in Ni-Fe systems.